1. Field of the Invention
This invention relates, in general, to photocopying apparatus and, more specifically, to vacuum conveyor systems for transporting sheets in copiers and printers.
2. Description of the Prior Art
Hard-copy producing apparatus, such as electrostatographic copiers and printers, often have a number of different process stations through which the produced sheets pass. In order to move the sheets between the stations, various types of transport or conveyor systems are used according to conventional practice. One type widely used in the industry employs conveyor belts which are disposed over a vacuum system which effectively attracts the sheets to the belts. These vacuum belt systems are sufficient in many types of apparatus, but, without changes, they offer little control over the speed of the sheet as it is moved by the conveyor system.
One frequent location for a vacuum transport system is between the transfer station, where the toner is transferred to the sheet, and the fixing or fusing station, where the transferred toner is melted and fused to the sheet. In some applications, the conveyor system between these stations even transports the sheet in an upside down orientation before the toner is fused to the paper or transparency sheet. While the vacuum transport system does a good job in some applications, conventional systems do not provide all of the versatility desired in some forms of hard-copy producing apparatus.
Matching the speed from one process station to another is important so that the paper sheet passes smoothly between the two process stations. In the case of a multipurpose machine which can produce color and monochrome copies or prints, the speed of the transfer station is often different than the speed of the fuser. For example, the transfer station may operate at a speed of 12 inches per second (ips) when sheets are delivered to the conveyor in both the monochrome and color modes. However, because of the need for multicolor transfer operations, the number of sheets delivered in the color mode on an average basis is less than that for the monochrome mode. This permits the use of a slower fuser speed to achieve better quality fusing. For transparencies, a fuser speed of 2 ips is typical. In order to accomplish smooth sheet transfers, some form of sheet slowing between the transfer and fusing stations is necessary because the fuser runs slower than the transfer station.
One method customarily used in the prior art involves reducing the vacuum on the belts to cause the sheet to slip on the belt and arrive at the fuser later than normal. This effectively slows the sheet and allows it to pass smoothly into the fusing station at substantially the same speed as the fuser. Such conditions and parameters as electrostatic charge on the paper, marking engine operating altitude, paper roughness, moisture content, and grain orientation all affect the amount of slippage of the sheet on the belts, in addition to the slip caused by the reduced vacuum. Thus, exact and precise control over the paper sheet is difficult to obtain with only the vacuum changes. In addition, if the vacuum becomes too low, maintaining the proper paper path can become difficult. This is especially important and troublesome when the paper is being transported along the bottom of the belts and may drop off if the vacuum becomes too low.
Therefore, it is desirable, and an object of this invention, to provide a variable speed transport system which can smoothly and predictably change the speed of a sheet being conveyed between two process stations in a copier, printer, or like apparatus.